This invention relates generally to a document reader device for use in facsimile data transmission or the like. In particular, it relates to a system for reducing noise in a large-size document reader device which has been recently developed as a counterpart to an array of MOS photodiodes or a CCD image sensor.
Large-size document reader devices (hereinafter referred to as a "large-size image sensors") are defined as document readers which are physically as large as the document or script to be read. Large-size image sensors "read" (or sense) a document image by employing an optical system, such as an array of either optical fibers or lenses, to focus the document image. This system is advantageous in that since it does not utilize a magnified image of the document, it has a reduced optical path for image focusing, thus making the system in general smaller in size than those systems in the prior art utilizing magnified images.
FIG. 1(a) shows an equivalent circuit of a conventional large-size image sensor, and FIG. 1(b) shows the structure of a photosensitive element 1. As shown in FIG. 1(b), the photosensitive element 1 comprises a photoconductive semiconductor thin film 21 composed of an amorphous material such as Se-As-Te, a -Si, or a polycrystal material such as CdS or CdSe. Thin film 21 is "sandwiched" between a thin film electrode 22 consisting of Al, Cr or Au, and a continuous transparent thin film electrode 23 consisting of either SnO.sub.2 or ITO. Referring to FIG. 1(a), the photosensitive element 1 is schematically represented as a photodiode 1a and a capacitor 1b, the capacitor 1b representing the inherent capacitance of the photosensitive element 1. A plurality of such photosensitive elements are arranged in an array of a density required to resolve a document or script, such as 8 elements/mm.
A charge stored in the capacitor 1b is discharged by the photodiodes 1a as a function of the amount of light which has condensed upon the photosensitive element 1. MOSFETs 2 are successively turned on at certain intervals by a shift register 3, to recharge the capacitors 1b. A current generated uon such recharging is detected by an output terminal 6 to read optical information.
With the large-size image sensor of the above construction, the photosensitive element 1 can be fabricated by forming the conductive thin film 22, the photoconductive thin film 21 and the transparent conductive film 23 on one substrate 24 composed of such materials as glass or ceramics, through typical processes such as evaporation, sputtering, or CVD. Since the MOSFETs 2 and the shift register 3 are independent devices, they cannot be fabricated integral to the photosensitive element 1. It is therefore necessary to mount the MOSFETs 2 and the shift register 3 on the substrate 24 or another substrate, and to connect them to the photosensitive element 1 by way of such methods as wire bonding. To effect such a connection, a wire 7 extends from the photosensitive element 1 to the corresponding MOSFET 2. The length of the wire 7 is in reality larger in proportion to the circuit elements than it appears to be in the equivalent circuit. In addition, signal line 8 has a length of several tens of centimeters in the large-size image sensor. Thus, capacitances between the wires or signal lines are quite large, and cause an amount of non-negligible noise to be introduced into the signal line 8, resulting in a lowered signal-to-noise ratio.
In an attempt to eliminate the foregoing difficulty, the present application has proposed a system which utilizes dummy MOSFETs to effect differential amplification, as described in an application entitled "Document Reader Device" filed on Nov. 10, 1981, in Japan and laid open on May 16, 1983 as No. 80865/83. That Japanese application is commonly assigned. As shown in FIG. 2 herein, the system comprises switching MOSFETs 2, dummy MOSFETs 10, and photosensitive elements 1. The MOSFETs 2 and 10 produce outputs which are differentially amplified such that the composite signal has a good signal-to-noise ratio. However, with two independent MOSFETs connected for each photosensitive element, the area needed to package all of the MOSFETs is twice as large as that required by the typical image sensor circuitry shown in FIG. 1(a). When the MOSFETs of the proposed system are fabricated on an IC chip, such an IC chip will therefore have a large packaging area. The increased packaging area presents an obstacle to the reduction in size of large-size document reader devices.
It is thus an object of the present invention to eliminate the decreased signal-to-noise ratios of large-size image sensors without appreciably increasing the size of the photodetector IC package.